1. Field of the Invention
The present invention relates to a scaled write head with high recording density and high data rate and, more particularly, to a write head where significant dimensions have been scaled downwardly proportionally except for thickness of a write coil layer and apex angle of an insulation stack.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. The write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A write gap layer between the first and second pole piece layers forms a magnetic gap at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the magnetic gap between the pole pieces. This field fringes across the magnetic gap for the purpose of writing information in tracks on moving media, such as the circular tracks on the aforementioned rotating disk or a linearly moving magnetic tape in a tape drive.
The read head includes first and second shield layers, first and second gap layers, a read sensor and first and second lead layers that are connected to the read sensor for conducting a sense current through the read sensor. The first and second gap layers are located between the first and second shield layers and the read sensor and the first and second lead layers are located between the first and second gap layers. The distance between the first and second shield layers determines the linear read density of the read head. The read sensor has first and second side edges that define a track width of the read head. The product of the linear density and the track density equals the areal density of the read head which is the bit reading capability of the read head per square inch of the magnetic media.
A significant factor in achieving gigabyte densities in computers has been increasing the track density of the write head. Track density is expressed in the art as tracks per inch (TPI) which is the number of tracks that the write head can write per inch of width of a rotating disk or linearly moving magnetic tape. In order to achieve high track density it is necessary that the second pole tip have a narrow width, which width is referred to in the art as track width. This pole tip is typically the second pole tip of the write head. Considerable research has been undertaken to provide submicron track widths for increasing the track density. The track width is measured between first and second side walls of the second pole tip at the ABS. It is important that these side walls be straight and well-formed so that the width is uniform between the side walls from the bottom to the top of the second pole tip. If these side walls are irregular the write head will write a poor magnetic impression into the rotating disk and the track width will be unpredictable from head to head.
A problem in constructing a highly defined narrow track width second pole tip has been "reflective notching" which occurs during the construction of the second pole tip at the ABS. The second pole piece layer is constructed after constructing the insulation stack which insulates one or more coil layers. After constructing the insulation stack a seed layer is sputter deposited on the insulation stack for frame plating the second pole piece layer. This seed layer, which is typically nickel iron (NiFe), is highly reflective to light. Next, a photoresist layer is spun on a wafer where the magnetic head is to be constructed. The photoresist layer has a height on top of the insulation stack and a depth in a pole tip region where the second pole tip is to be constructed. Since the photoresist layer tends to be planarized, due to the spinning operation, the depth of the photoresist in the pole tip region is greater than the height of the photoresist above the insulation stack. Next, the photoresist layer is photo-imaged exposing a region of the photoresist layer that is to be removed. This region is then removed by a developer which provides an opening in the photoresist layer where the second pole piece and second pole tip are to be formed. Unfortunately, light can be reflected into regions adjacent the first and second side wall sites of the second pole tip which, upon developing, removes portions of the photoresist mask adjacent the first and second side walls. These portions are irregular in shape and, upon plating the second pole piece and second pole tip, causes the second pole tip to have irregularly shaped side walls. This unwanted light reflection occurs behind a flare point of the second pole piece layer. The flare point is where the second pole piece layer first commences to widen after the pole tip region. The sloping surfaces of the insulation stack behind the flare point are the reflective regions which reflect the unwanted light adjacent the first and second side walls of the second pole tip. Accordingly, the prior art tends to locate the flare point further from the ABS so that the amount of light reflection is minimized. Unfortunately, the longer the pole tip region between the flare point and the ABS the greater the magnetic saturation of the second pole tip, which reduces the recording strength of the write head. Factors bearing upon the degree of reflective notching are the height of the insulation stack, the proximity of the insulation stack to the ABS, the location of the flare point and the apex angle. The apex angle is the angle that the insulation stack takes with respect to the plane of the write gap layer. As this angle becomes less than 45.degree. there is less light reflected into the regions adjacent the second pole tip region. The height of the insulation stack also bears on the thickness of the photoresist in the pole tip region. The greater this thickness the greater the dispersion of light during the light exposure step, which causes poor resolution of the light at the bottom of the photoresist where the side walls are to be formed. Reflective notching is fully explained in commonly assigned U.S. Pat. No. 5,798,897 which is incorporated by reference herein. Reflective notching has been a serious problem in obtaining high track width densities.
Another factor bearing upon high density magnetic disk drives is the linear recording density of the write head. Linear recording density is measured in bits per inch (BPI), which is the number of bits that can be written by the write head per linear length of a track along the magnetic disk. Linear bit density is directly dependent upon the length of the write gap between the first and second pole tips. This length is equal to the thickness of the write gap layer. A major problem in obtaining thin write gap layers has been the processing steps in constructing the insulation stack, which steps typically are subsequent to construction of the write gap layer. After construction of the write gap layer the first insulation layer of the insulation stack is constructed on top of the first pole piece layer, the write coil layer is frame plated on the first insulation layer, the second insulation layer is formed on the write coil layer and a third insulation layer is typically employed for smoothing out the ripples of the second insulation layer. The insulation layers are hard baked photoresist. Photoresist is spun on the wafer, light exposed in areas that are to be removed by a developer. After constructing all of the insulation layers they are then hard baked so that they become hard. The write coil layer is constructed by frame plating. A photoresist layer is spun on the head, light exposed in the regions to be removed and then developed. After frame plating the write coil layer the photoresist is removed and the wafer is ion beam milled in order to remove all portions of the seed layer except the seed layer underneath the write coil layer. It is this ion beam milling step, in particular, that reduces the thickness of the write gap layer causing this thickness to be unreliable. Accordingly, the prior art tends to deposit a thicker write gap layer to account for the ion beam milling operation.
The product of the track density and the linear density of the second pole tip determine the overall recording density of the write head. This product is known in the art as "areal density". Another factor rating the capability of the write head is its data rate. The data rate of the write head is its frequency of writing bits of information into the circular tracks of the rotating disk. The data rate is inversely proportional to the inductance of the write head. Inductance is directly proportional to the length, width and thickness of the first and second pole piece layers. Accordingly, it is desirable to reduce the size of these layers in order to increase the data rate.
Another concern with a write head is the heat generated by it while operating in a disk drive. A write current is conducted through the write coil at high frequencies which generates heat. The amount of heat generated is inversely proportional to the cross section of the turns of the coil. Because of the different coefficients of expansions of materials in the write head, such as photoresist for the insulation stack and aluminum oxide (Al.sub.2 O.sub.3) for an overcoat layer covering the write head, the heat can cause a protrusion problem at the air bearing surface. The heat causes the insulation stack to expand more than the overcoat layer which results in the overcoat layer being pushed forward beyond the ABS. While it is desirable to decrease the thickness of the write coil layer in order to reduce the height of the insulation stack so as to minimize reflective notching, this is difficult to do because of the heat generated by the smaller coil layer.